The present invention generally relates to the field of compensating for undesired temperature variation of a capacitor. More specifically, the present invention relates to a compensating circuit and method for compensating for the undesired temperature variation of the capacitance of capacitor which, for example, can comprise a pressure sensing capacitive element.
In prior capacitive pressure sensors, a capacitive pressure sensing element, typically comprising a parallel plate capacitor, has the spacing between its plates varied in accordance with sensed pressure. This changes the capacitance of the sensing element, and this capacitance change is utilized to develop an electrical signal which varies in accordance with sensed pressure. However, in addition to the capacitance of the sensing element varying in accordance with sensed pressure, typically this capacitance also varies in accordance with temperature. Various types of temperature compensation circuits have been utilized to provide temperature compensation for this undesired temperature variation.
Some capacitive temperature compensation circuits utilize temperature varying resistors, such as thermistors, to produce a temperature-varying electrical signal which is then combined with a temperature-varying signal produced by a capacitor, such as a capacitive sensing element. However, thermistors generally have substantial stability problems, especially as they age. Also, typically only thermistors with negative temperature coefficients are usable for linear compensation applications, and positive temperature coefficient thermistors are only usable for switching applications. Thus proper circuit design for capacitor compensation circuits which use thermistors may be difficult. In addition, the providing of an additional temperature varying compensation signal due to a thermistor typically results in a complex temperature compensation system.
Some prior capacitor temperature compensation systems have attempted to reduce the temperature variation of a capacitor by directly connecting, in parallel to that capacitor, a compensation capacitor having a temperature coefficient of capacitance (TCC) polarity opposite to the TCC polarity of the capacitor to be compensated. However, typically such prior systems require matching of the magnitude of the TCC of the capacitor to be compensated for to the magnitude of the TCC of the compensation capacitor such that the two TCC's essentially cancel each other out. Achieving this in the real world presents a problem since typically the TCC of a capacitor cannot be readily adjusted. Thus these prior compensation systems are deficient in that they do not readily allow a selective adjustment of the temperature compensation circuit so as to provide a desired effective composite temperature variation of a capacitive sensing element and a compensation capacitor, wherein typically a zero composite temperature variation is desired.